1. Field of the Invention
This invention relates generally to the field of integrated circuits and, more particularly, to a resistor circuit with a DC voltage control.
2. Description of the Related Art
Resistors are frequently incorporated into integrated circuits. One well known technique for fabricating and using resistors within an integrated circuit is to deposit a layer or strip of polysilicon material onto a semiconductor substrate. The polysilicon strip, with contacts used to form electrical connections, forms a passive resistor (also referred to in the art as a polysilicon resistor). The polysilicon material has a known resistivity, expressed in .OMEGA./cm.sup.2, and the resistance of the polysilicon resistor is directly related to the length of the polysilicon strip used to form the resistor.
FIG. 1a illustrates a polysilicon resistor 10 formed from a layer or strip of polysilicon material 12. The resistor 10 has two contacts 14, 16 used to form electrical connections between the resistor 10 and other circuit elements. As illustrated in FIG. 1a, the polysilicon resistor 10 is connected between an input signal IN and a supply voltage V.sub.SD. FIG. 1b illustrates the electrical circuit of FIG. 1a including polysilicon resistor 10 of FIG. 1a represented by its known impedance R.sub.12.
Although the use of polysilicon resistors provides an attractive and convenient method for incorporating resistors within integrated circuits, the use of such resistors is not without problems. One known problem, for example, is that the sheet resistance of the polysilicon material varies. The variation can be as large as .+-.30 percent of the expected impedance. The sheet resistance, and thus, the impedance of the polysilicon resistor, is effected by variations in the manufacturing process. In addition, the impedance of the polysilicon resistor is effected by operating temperature and power supply voltage variations which are difficult to control when the polysilicon resistor is in operational use.
This is a particular problem in applications requiring the polysilicon resistors to be maintained at a precise impedance. In high speed signal transmission environments, for example, the impedance of an integrated circuit that is sending or receiving a signal over a signal path must be a precise value. When a signal exits an integrated circuit, travels an appreciable distance along the signal path and enters another integrated circuit, signal reflections can be experienced from impedance discontinuities at any point along the signal path. These undesirable reflections result in reduced noise immunity and increase the time for the signal to become, and remain, valid at the far end of the signal path. It is well known, however, that when the signal path is viewed as a transmission line with a characteristic impedance, undesirable reflections are eliminated when the transmission line is terminated at the sending and/or receiving ends with an impedance having a value equal to the characteristic impedance of the transmission line.
FIGS. 1a and 1b illustrate the use of a polysilicon resistor 10 to terminate a transmission line 20 in a high speed transmission environment. The length of the polysilicon material 12 used to create the polysilicon resistor 10 is chosen to match the impedance of the transmission line 20. However, with possible variations as large as .+-.30 percent of the desired impedance, undesirable reflections and transmission problems may occur.
Accordingly, there is a desire and need for an integrated circuit containing polysilicon resistors with well controlled impedances to overcome resistor impedance variations which may be caused by variations in the manufacturing process, operating temperature or operating power supply voltage.